Printed circuit boards are almost universally used in the electronics industry to mount electrical components and to interconnect these components by means of copper conductor paths printed, or laid, on the board surface. As components are being created which perform more functions while also designed in more compact arrangements thus allowing more components to be positioned on a board surface, the number of copper conductor paths, or traces, required to operate these miniaturized powerful components has not only grown substantially, but the path layouts have also become more complex. Printed circuit board manufacturers have attempted to meet these design demands for more component circuit configurations per board by incorporating new manufacturing techniques which have permitted thinner copper conductors to be laid on the board surface. These printed wiring boards having thinner conductors, also known as fine lines, allow higher densities of conductors to be applied to a board surface by laying conductors having a width of 0.0125" (31.7 mm) or less between conductors. Another technique used to increase the number of circuit configurations per board is to place circuits on each side of the board and to use plated-thru holes as conductor paths between the two sides. To use plated-thru holes as conductors, or as component lead receptacles, it is necessary that a solder compatible material, perferably solder itself, be plated on the walls of the holes and on adjacent terminal pad areas. For this to be done reliably, the usual method has been to apply solder to all areas of the copper conductor traces on both sides of the board and this includes both the thru-hole traces and the fine line conductor traces or paths.
However problems arise after components have been inserted into the fine line circuit board and it is passed through a soldering station, such as a solder wave bath. Heat from the soldering operation causes the solder layer on the fine line conductor paths to melt and flow to form short circuits with adjacent conductor paths. In the past, to prevent the solder from melting and flowing, the boards have been treated with a gelatinous mixture called solder mask. To operate effectively, the solder mask must completely cover each conductor path and also cover the board surfaces between the paths to inhibit solder flow between the paths. However, when the soler mask is applied to boards having fine line conductor paths, the space between the paths is so narrow that the gelatinous fluid can not flow down between the conductor paths to cover the interpath board surface but instead either forms a bridge between the paths and over the interpath surface, or is missing entirely from the interpath surface. Thus when the board is passed through the solder station the solder layer on the conductor path becomes heated and flows under the bridge of soldermask or across the non covered interpath surface to effect a short circuit between adjacent paths. Since this prior art method does not prevent solder short circuits, some manufacturers have had to resort to manual final inspection and solder touch up which is costly and time consuming.
There is a need therefore for a method of manufacturing printed wiring boards which prevents short circuits from forming between conductor paths.